The present invention relates to a communications system for data transmission over a time division duplex (TDD) frequency channel, for example in a digital cordless telephone system having time division multiple access (TDMA) protocol.
FIGS. 1 and 2 of the accompanying drawings illustrate respectively an example of a digital cordless telephone system and the channel and message structure.
The digital cordless telephone system comprises a plurality of primary or base stations PS of which four, PS1, PS2, PS3 and PS4, are shown. Each of the primary stations is connected by way of a respective wideband landline link 10, 11, 12 and 13, capable of carrying data at a rate of say 1.152 Mbits/sec. to cordless telephone system controllers 14 and 15. The system controllers 14 and 15 are, in the illustrated embodiment, connected to the PSTN which is constituted by an ISDN (Integrated Services Digital Network) link.
The system further comprises a large plurality of secondary stations SS some of which, SS1, SS2, SS4 and SS5, are hand portable and are used for digital time division duplex speech communication only. Others, for example SS3 and SS6, are data terminals which also are capable of duplex data communication. Duplex communication between the secondary stations within an area covered by a system controller and/or the PSTN is by way of radio through the primary stations PS. Accordingly the primary and secondary stations each comprise a radio transmitter and receiver.
Referring to FIG. 2, the illustrated system has five radio channels, hereinafter referred to as frequency channels C1 to C5, each capable of carrying digitised speech or data at 1.152 Mbits/sec. The adjacent frequency channel separation is 1.728 MHz. Each frequency channel is divided in the time domain into 10 ms frames. Each frame is divided into 24 time slots (or physical channels) of which the first twelve F1 to F12 are allocated for transmission in a forward direction, that is from a primary station to a secondary station, and the second twelve R1 to R12 are allocated for transmission in the reverse direction. The forward and reverse time slots are twinned, that is, the correspondingly numbered forward and reverse time slots, for example F4, R4, comprise a twin which hereinafter will be referred to as a duplex voice channel. In setting-up a call between a primary and a secondary station, a duplex voice channel is assigned to the transaction. The assignment of the duplex voice channel in any of the frequency channels C1 to C5 is by the method of dynamic channel allocation whereby a secondary station taking account of its radio environment negotiates with the primary station for access to the best duplex voice channel currently available under the control of the primary station. The system controller 14 or 15 will effect error detection and correction to data received by any one of the primary stations to which it is connected. Error control of the digitised speech is performed by the primary stations.
The general structure of a message is also shown in FIG. 2. The message structure comprises two bytes of preamble 16, two bytes of a synchronisation sequence 18, eight bytes of signalling data 20 and forty bytes of digitised speech or data 22. The digitisation rate and data rate is 32 kbits/sec. Both the primary and secondary stations include a buffer to compress the 32 kbits/sec. data to bursts of data at 1.024 Mbits/sec. so that it is suitable for transmission.
The basic protocol for a transmission which is to be initiated by a secondary station SS is for it to listen to all the forward physical channels in each of the frequency channels C1 to C5 and ascertain which duplex voice channels are busy and idle and the relative signal quality in these duplex voice channels. From the information derived the secondary station determines what it believes is the best duplex voice channel and transmits in the reverse physical channels of the duplex voice channel to a particular primary station PS. The signalling details 20 in the message together with the details 22 in the initial transmission are decoded and passed to the system controller 14 or 15 which sets-up the fixed network connection. The primary station confirms that the particular duplex voice channel has been assigned to the transaction.
In the forward direction, the primary stations send paging messages to the addressed secondary stations in say every sixteenth frame. Such an arrangement enables the secondary stations to "sleep" during at least the intervening fifteen frames thereby economising on power. An addressed secondary station in response to a paging request addressed to it will, unless a duplex voice channel has been assigned, transmit on the reverse time slot of the best duplex voice channel. As a general rule the system protocol will give priority to speech over data.
For data transmission at data rates of the order of 32 kbits/sec. then transmissions in the forward and reverse directions can proceed in a similar manner to speech. However it is not unusual for a secondary station to generate batches of data at rates in excess of 32 kbits/sec. and also it is desirable for the cordless system to handle higher data rate services such as 2B+D Integrated Services Digital Network (ISDN) which operates at 144 kbits/sec. B equals 64 kbits/sec. and is suited to send fax messages, digitised speech and certain other services over the PSTN (public switched telephone network) and D equals 16 kbits/sec. and is used for signalling involved with call set-up and other routine tasks. Since data rates for ISDN and for other types of data transmission, such as graphics, exceed the capacity of a time slot then either buffering could be used since data tends to be transmitted in bursts or one or more additional duplex voice channels could be assigned to the transmission of a high data rate message. Allowing for retransmissions as a result of detected errors then it is conceivable that even more channel capacity will be required to complete a transaction. If a data transaction uses a disproportionately large amount of each frame then this will reduce access to the duplex voice channels by other users wanting to make speech calls. There are applications such as cordless video phones and computer communications where rapid access to one or more duplex voice channels is required by the nature of the data to be transacted but after having grabbed the additional duplex voice channels they are not retained for longer than is necessary after which they are available for other system users. Dynamic channel allocation imposes an undesired time overhead and it is an object of the present invention to reduce this overhead.